1. Field of the Invention
The present invention is directed to equipment for testing microcircuits.
2. Description of the Related Art
The invention pertains to improvements to equipment for testing microcircuits. The manufacturing processes for microcircuits cannot guarantee that every microcircuit is fully functional. Dimensions of individual microcircuits are microscopic and process steps very complex, so small or subtle failures in a manufacturing process can often result in defective devices.
Mounting a defective microcircuit on a circuit board is relatively costly. Installation usually involves soldering the microcircuit onto the circuit board. Once mounted on a circuit board, removing a microcircuit is problematic because the very act of melting the solder for a second time ruins the circuit board. Thus, if the microcircuit is defective, the circuit board itself is probably ruined as well, meaning that the entire value added to the circuit board at that point is lost. For all these reasons, a microcircuit is usually tested before installation on a circuit board.
Each microcircuit must be tested in a way that identifies all defective devices, but yet does not improperly identify good devices as defective. Either kind of error, if frequent, adds substantial overall cost to the circuit board manufacturing process.
Microcircuit test equipment itself is quite complex. First of all, the test equipment must make accurate and low resistance temporary and non-destructive electrical contact with each of the closely spaced microcircuit contacts. Because of the small size of microcircuit contacts and the spacings between them, even small errors in making the contact will result in incorrect connections. Connections to the microcircuit that are misaligned or otherwise incorrect will cause the test equipment to identify the device under test (DUT) as defective, even though the reason for the failure is the defective electrical connection between the test equipment and the DUT rather than defects in the DUT itself.
A further problem in microcircuit test equipment arises in automated testing. Testing equipment may test 100 devices a minute, or even more. The sheer number of tests cause wear on the tester contacts making electrical connections to the microcircuit terminals during testing. This wear dislodges conductive debris from both the tester contacts and the DUT terminals that contaminates the testing equipment and the DUTs themselves.
The debris eventually results in poor electrical connections during testing and false indications that the DUT is defective. The debris adhering to the microcircuits may result in faulty assembly unless the debris is removed from the microcircuits. Removing debris adds cost and introduces another source of defects in the microcircuits themselves.
Other considerations exist as well. Inexpensive tester contacts that perform well are advantageous. Minimizing the time required to replace them is also desirable, since test equipment is expensive. If the test equipment is off line for extended periods of normal maintenance, the cost of testing an individual microcircuit increases.
Test equipment in current use has an array of test contacts that mimic the pattern of the microcircuit terminal array. The array of test contacts is supported in a structure that precisely maintains the alignment of the contacts relative to each other. An alignment template or board aligns the microcircuit itself with the test contacts. The test contacts and the alignment board are mounted on a load board having conductive pads that make electrical connection to the test contacts. The load board pads are connected to circuit paths that carry the signals and power between the test equipment electronics and the test contacts.
So-called “Kelvin” testing refers to a process where each microcircuit terminal contacts two test contacts. A preliminary part of the test procedure measures the resistance between the two test contacts. If this value is high, one or both of the two test contacts are not making good electrical contact to the microcircuit terminal. If the possibility of high resistance at this interface will affect the accuracy of the actual testing of the microcircuit performance, then the issue can be addressed according to the provisions of the testing protocol.
In the appended drawings, the form factors for the various components shown are not to scale where it may make it easier for the reader to understand the invention. Where relevant or helpful, the description includes representative dimensions.
One particular type of microcircuit often tested before installation has a package or housing having what is commonly referred to as a ball grid array (BGA) terminal arrangement. FIGS. 1 and 2 show an example of a BGA package type of microcircuit 10. Such a package may have the form of a flat rectangular block, with typical sizes ranging from 5 mm to 40 mm on a side and 1 mm thick.
FIG. 1 shows microcircuit 10 with a housing 13 enclosing the actual circuitry. Signal and power (S&P) terminals 20 are on one of the two larger, flat surfaces, surface 14, of housing 13. Signal and power (S&P) terminals 20 surround a projection 16 on surface 14. Typically, terminals 20 occupy most of the area between the surface 14 edges and spacer 16 rather than only a portion of the area as is shown in FIG. 1. Note that in some cases, spacer 16 may be an encapsulated chip or a ground pad.
FIG. 2 shows an enlarged side or elevation view of terminals 20 as they appear with surface 14 on edge. Each of the terminals 20 includes a small, approximately spherical solder ball that firmly adheres to a lead from the internal circuitry penetrating surface 14, hence the term “ball grid assembly.” FIG. 2 shows each terminal 20 and spacer 16 projecting small distances away from surface 14, with the terminals 20 projecting farther from surface 14 than the spacers 16. During assembly, all terminals 20 are simultaneously melted, and adhere to suitably located conductors previously formed on the circuit board.
Terminals 20 may be quite close to each other. Some have centerline spacings of as little as 0.5 mm, and even relatively widely spaced terminals 20 are still around 1.5 mm apart. Spacing between adjacent terminals 20 is often referred to as “pitch.”
In addition to the factors mentioned above, BGA microcircuit testing involves additional factors.
First, in making the temporary contact with the ball terminals 20, the tester should not scratch or otherwise mark the S&P terminal surfaces that contact the circuit board, since such a mark may affect the reliability of the solder joint for that terminal.
Second, the testing process is more accurate if the length of the conductors carrying the signals is kept short. An ideal test contact arrangement has short signal paths.
Third, solders commonly in use today for BGA terminals are mainly tin for environmental purposes. Tin-based solder alloys are likely to develop an oxide film on the outer surface that conducts poorly. Older solder alloys include substantial amounts of lead, which do not form oxide films. The test contacts must be able to penetrate the oxide film present.
BGA test contacts currently known and used in the art employ spring pins made up of multiple pieces including a spring, a body and top and bottom plungers.
United States Patent Application Publication No. US 2003/0192181 A1, titled “Method of making an electronic contact” and published on Oct. 16, 2003, shows microelectronic contacts, such as flexible, tab-like, cantilever contacts, which are provided with asperities disposed in a regular pattern. Each asperity has a sharp feature at its tip remote from the surface of the contact. As mating microelectronic elements are engaged with the contacts, a wiping action causes the sharp features of the asperities to scrape the mating element, so as to provide effective electrical interconnection and, optionally, effective metallurgical bonding between the contact and the mating element upon activation of a bonding material.
According to United States Patent Application Publication No. US 2004/0201390 A1, titled “Test interconnect for bumped semiconductor components and method of fabrication” and published on Oct. 14, 2004, an interconnect for testing semiconductor components includes a substrate, and contacts on the substrate for making temporary electrical connections with bumped contacts on the components. Each contact includes a recess and a pattern of leads cantilevered over the recess configured to electrically engage a bumped contact. The leads are adapted to move in a z-direction within the recess to accommodate variations in the height and planarity of the bumped contacts. In addition, the leads can include projections for penetrating the bumped contacts, a non-bonding outer layer for preventing bonding to the bumped contacts, and a curved shape which matches a topography of the bumped contacts. The leads can be formed by forming a patterned metal layer on the substrate, by attaching a polymer substrate with the leads thereon to the substrate, or by etching the substrate to form conductive beams.
According to U.S. Pat. No. 6,246,249 B1, titled “Semiconductor inspection apparatus and inspection method using the apparatus” and issued on Jun. 12, 2001 to Fukasawa, et al., a semiconductor inspection apparatus performs a test on a to-be-inspected device which has a spherical connection terminal. This apparatus includes a conductor layer formed on a supporting film. The conductor layer has a connection portion. The spherical connection terminal is connected to the connection portion. At least a shape of the connection portion is changeable. The apparatus further includes a shock absorbing member, made of an elastically deformable and insulating material, for at least supporting the connection portion. A test contact element of the invention for making temporary electrical contact with a microcircuit terminal comprises at least one resilient finger projecting from an insulating contact membrane as a cantilevered beam. The finger has on a contact side thereof, a conducting contact pad for contacting the microcircuit terminal.
In U.S. Pat. No. 5,812,378, titled “Microelectronic connector for engaging bump leads” and issued on Sep. 22, 1998 to Fjelstad, et al., a connector for microelectronic includes a sheet-like body having a plurality of holes, desirably arranged in a regular grid pattern. Each hole is provided with a resilient laminar contact such as a ring of a sheet metal having a plurality of projections extending inwardly over the hole of a first major surface of the body. Terminals on a second surface of the connector body are electrically connected to the contacts. The connector can be attached to a substrate such a multi-layer circuit panel so that the terminals on the connector are electrically connected to the leads within the substrate. Microelectronic elements having bump leads thereon may be engaged with the connector and hence connected to the substrate, by advancing the bump leads into the holes of the connector to engage the bump leads with the contacts. The assembly can be tested, and if found acceptable, the bump leads can be permanently bonded to the contacts. According to United States Patent Application Publication No. US 2001/0011907 A1, titled “Test interconnect for bumped semiconductor components and method of fabrication” and published on Aug. 9, 2001, an interconnect for testing semiconductor components includes a substrate, and contacts on the substrate for making temporary electrical connections with bumped contacts on the components. Each contact includes a recess and a support member over the recess configured to electrically engage a bumped contact. The support member is suspended over the recess on spiral leads formed on a surface of the substrate. The spiral leads allow the support member to move in a z-direction within the recess to accommodate variations in the height and planarity of the bumped contacts. In addition, the spiral leads twist the support member relative to the bumped contact to facilitate penetration of oxide layers thereon. The spiral leads can be formed by attaching a polymer substrate with the leads thereon to the substrate, or by forming a patterned metal layer on the substrate. In an alternate embodiment contact, the support member is suspended over the surface of the substrate on raised spring segment leads.